adding builtin support

I am struggling again to add builtin support for Solaris to a package.
This time I'd like to use native openssl (actually the on in /usr/sfw)
instead of security/openssl. I have severe performance problems in a
productive radius server.

Maybe someone can give me a hint on what to check/fix next. What is
missing to set BUILDLINK_PREFIX.openssl correctly? What variables are
actually checked when a depending package decides to take the native or
the pkgsrc version?